Wastewater treatment and treated effluent goals and standards have become increasingly stringent for the economical removal of wastewater components such as total suspended solids (TSS), biological oxygen demand (BOD), nitrogen (as nitrate and ammonia) and phosphorous from large volumes of municipal and industrial wastewater. Activated sludge systems of either the continuous flow type in which an influent stream is continuously treated and continuously discharged through one or more treatment zones, or the sequencing batch reactor type in which a continuous influent stream is sequentially treated and intermittently discharged, are conventionally used for wastewater treatment. In such activated sludge treatment systems, treatment microorganisms are concentrated in the treatment system in order to more rapidly remove the wastewater impurities, including BOD, nitrogenous, and phosphorous components of the wastewater. The highly diverse, mixed cultures utilized in such activated sludge wastewater treatment systems for biological removal of BOD, nitrogen and phosphorous include ordinary heterotrophs (which can consume organic wastewater components to produce carbon dioxide and reduce BOD, as well as mediate denitrification), autotrophs (which mediate nitrification in consuming nitrogenous wastewater components) and phosphotrophs (which can accumulate polyphosphates in consuming phosphorous-containing wastewater components).
The various types of microorganisms in activated sludge cultures typically utilize different nutrient, oxygenation and other conditions for optimum removal of different wastewater components. The organic materials in the wastewater are consumed by "activated sludge" microorganisms for both energy and cell synthesis, driven by biological oxidation-reduction reactions involving transfer of electrons from a wastewater component to be oxidized (the electron donor) to an oxidizing material (the electron acceptor). Heterotrophic metabolism utilizes organic wastewater components as electron donors, while autotrophic metabolism utilizes inorganic wastewater components as electron donors. In aerobic systems in which the wastewater is aerated, oxygen is utilized by "activated sludge" microorganisms as the terminal electron acceptor. In anoxic systems, the oxygen is substantially depleted, and "activated sludge" microorganisms utilize nitrates and nitrites as the primary terminal electron acceptors. Under anaerobic conditions, oxygen, nitrate and nitrite components are substantially depleted, and carbonates and sulfates serve as primary terminal electron acceptors in the cell reactions (M. G. Mandt and B. A. Bell "Oxidation Ditches", 169 pgs., 1982, Ann Arbor Science Publishers). It should be noted that different microorganisms and/or metabolic pathways may predominate under such different aerobic, anoxic and anaerobic conditions.
Sequencing batch reactors such as described in U.S. Pat. No. 4,596,658 to Mandt, are conventionally utilized for wastewater treatment to provide high quality effluent by subjecting a given volume of wastewater to a predetermined sequence of different treatment steps in batch mode, in the same batch reactor equipment. In this regard, a volume of wastewater may typically be introduced as a continuous or discontinuous feed stream into a sequencing batch reactor treatment system and subjected to extensive mixing and aeration for a predetermined period of time to provide biological oxidation, consumption or other removal of wastewater components. The mixing and aeration may subsequently be stopped and the wastewater maintained in a quiescent state in the same treatment zone to permit wastewater solids, including microbiological treatment organisms, to settle in the reactor. A clarified portion of the treated wastewater may be subsequently removed from the upper portion of the reactor, which in turn may be conducted to subsequent treatment and discharge steps. Additional wastewater which is to be treated may then be introduced into the sequencing batch reactor, and the cycle repeated. For many wastewater treatment applications, sequencing batch reactors may provide a number of advantages over older type continuous flow treatment systems in terms of expense, physical area and operating energy requirements. However, although sequencing batch reactors have proven to be efficient, flexible and economic wastewater treatment systems, further improvements which could increase the processing efficiency, and/or optimize treatment conditions, such as anoxic and aerobic treatment conditions, for wastewater component removal would be desirable. Such improved sequencing batch reactor methods and apparatus would be desirable which would be simple and effective in operation, which would permit enhancement and synergistic interaction of anoxic and aerobic treatment conditions for assisting wastewater component removal, and which would enhance the utility and cost effectiveness of sequencing batch reactors for wastewater treatment. Accordingly, it is an object of the present invention to provide such improved methods and apparatus and sequencing batch reactor systems which utilize such methods and apparatus. These and other objects of the invention will become more apparent from the following detailed description and the accompanying drawings.